KS3 · Biology

Diffusion through the cell membrane

Right now, oxygen is getting into every cell in your body. Not one cell is pumping it, sucking it in or choosing it. So how does it get in?

Follow the urea

Out of a liver cell and into the blood

Liver cells make a waste product called urea. The line is a sketch of how much urea there is: lots on the cell side, much less on the blood side. Tap each zone from left to right, and before you reach the end, predict which way the urea will go.

Zone 1 of 3

Cytoplasm of the liver cell

The liver cell makes urea, so there are lots of urea particles in its cytoplasm. Lots of particles in a space means a high concentration. Keep this in mind: this is the crowded side.

  • Urea is made here
  • Higher concentration of urea

Here's the trick: read the two sides, not the cell. Urea is more concentrated in the cytoplasm and less concentrated in the blood, so its net movement is from the cytoplasm, through the holes in the membrane, into the blood. That's moving down the concentration gradient: downhill on this sketch.

Watch out: The slope is only a picture of more on one side and less on the other. Nothing rolls down it and nothing pushes the urea out. So what does move it? That's the next step.

Zoom in on the particles

Imagine you could see the particles themselves.

A membrane has holes that some particles fit through. There are many more of these particles on the left of the membrane than on the right. What happens next?

Through the holes, or not?

What can get through the membrane?

Pick a substance, then choose where it belongs. Can its particles fit through the small holes in the cell membrane?

Still to sort

Fits through the holes (0)

These particles can diffuse into or out of the cell.

Where the line is: It comes down to size: particles small enough for the holes can cross.

Too big to get through (0)

These particles can't diffuse through the membrane.

Where the line is: Particles too big for the holes can't cross, however many of them there are.

4 of 4 still to sort.

Watch out: Selective doesn't mean the cell is picking and choosing. The holes do the sorting, simply by size.

Why it matters

?

Reason it through

Why must oxygen get into your cells, and carbon dioxide get out?

Link 1 of 4

First link · your turn

What do your cells use oxygen for?

2
Locked — reveal the link above first
3
Locked — reveal the link above first
4
Locked — reveal the link above first

What do you think?

How does oxygen get into a cell?

Oxygen is getting into the cells of your body right now. Here are four ways people explain it.

Which is closest to what you think right now?
How sure are you?

WHAT YOU'VE LEARNED

A quick recap of today's lesson.

How substances get into and out of cells, without the cell lifting a finger.

What you need to know

  • Diffusion is the net movement of particles of a substance down a concentration gradient.
  • Particles move randomly both ways, but more move from higher to lower concentration.
  • The cell membrane has small holes: some particles fit through, others (like protein molecules) are too big.
  • Diffusion is passive: cells don't choose, breathe or suck in substances.

The big picture

Substances get into and out of cells by diffusion: the net movement of particles down a concentration gradient, from where there are more of them to where there are fewer. It happens because particles are always moving randomly. The cell membrane has small holes that let some particles through but not others, so it is selectively permeable. And diffusion is passive: the cell doesn't choose, breathe or suck anything in.

Key points

1All substances are made of particles. The particles in the cytoplasm are constantly moving randomly, and that movement is what causes diffusion.
2To find the direction, compare the two sides: net movement goes from the side with the higher concentration to the side with the lower concentration.
3Liver cells make urea, so urea is more concentrated in their cytoplasm than in the blood. It diffuses out through the membrane holes into the blood.
4The cell membrane controls what enters and leaves the cell, and it is flexible, so the cell can change size and shape.
5The membrane is selectively permeable: its holes let some substances through but are not so big that everything in the cell escapes, which keeps conditions inside the cell relatively constant.
6Oxygen, needed for respiration, diffuses into animal cells. Carbon dioxide, a waste product of respiration that is toxic if it builds up, diffuses out into the blood.

Worked example

Problem

Oxygen diffuses from outside an animal cell into its cytoplasm. What does this tell you about the oxygen concentration on each side of the membrane? Explain your answer.

⚠ Watch out

Writing that in diffusion particles 'move from high to low concentration', as if every particle only goes that way. Particles cross in both directions. It's the net movement, the overall movement, that goes from higher to lower concentration.

🧠

Memory hook

Crowded side to quiet side. The particles do all the work; the membrane just decides who's small enough to get through.

✓

Check yourself

Cover the page. A cell has more carbon dioxide inside it than in the blood outside. Explain which way it diffuses, how it crosses the membrane, and what the cell must do. (Trick question!)

Flashcards

(14)
What is diffusion?
The net movement of particles of a substance down a concentration gradient.
What does 'net movement' mean?
The overall movement of particles from one area to another. Particles go both ways, but more go one way.
What causes diffusion?
The constant, random movement of particles in all directions.
What is a concentration gradient?
A difference in concentration between two areas, such as more urea in a liver cell than in the blood.
What does the cell membrane do?
It surrounds the cell and controls what can enter and leave.
Why can a cell change its size and shape?
Its cell membrane is flexible.
How do particles get through the cell membrane?
Through small holes in it, if they are small enough to fit.
Name particles too big to diffuse through the cell membrane.
Protein molecules.
What does 'selectively permeable' mean?
Permeable: lets things pass through. Selectively: lets some things through and not others.
How do the membrane's holes keep conditions inside the cell relatively constant?
They are big enough to let some substances through, but not so big that everything in the cell escapes.
Why does oxygen diffuse into animal cells?
The cells need oxygen for respiration.
Why must carbon dioxide leave cells?
It is a waste product of respiration and is toxic if it builds up, so it diffuses out into the blood.
Which way does urea diffuse from a liver cell, and why?
From the cytoplasm into the blood. Liver cells make urea, so it is more concentrated in the cytoplasm than in the blood.
Does a cell have to do anything to make diffusion happen?
No. Diffusion is passive: particles diffuse through the membrane down a concentration gradient by themselves.

Tap any card to flip it, or use Study as deck to go through them one at a time. In the full lesson these run as a spaced-repetition deck — you rate each card Hard, Good or Easy and the tricky ones keep coming back until they stick.

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How this lesson was checked. This KS3 Biologylesson was published through Lightbulb Learning's human-designed editorial process — the educational standards, accuracy rules and publication checks it must pass were authored and approved by Philip Halpin. It passed subject-specific assessment, automated educational checks and technical publication verification before going live (publication checks completed 29 September 2026). Published pages are monitored, human spot-checking is ongoing across the lesson library, and anything found wrong is corrected or withdrawn. How our lessons are made and checked. Spotted a mistake? Email hello@lightbulblearning.co and we'll review it.